α-Alumina powder has been widely used as an abrasive, a material for sintered body and a filler. In the application to abrasive the smoother surface roughness is required. Particularly when a precision component such as a hard disk or a magnetic head is subjected to grind for polishing finish, a more finely milled α-alumina having a primary particle diameter not more than 100 nm has been required so as to lessen surface roughness after polishing finish.
In a conventional α-alumina powder, however, the content of the α-phase which have higher hardness is small and this raise a problem of small grinding rate and thus a fine powder of α-alumina having an high content of α-phase has been required.
In the application to the material for sintered body, a very fine α-alumina powder has been required since the strength of the sintered body is expected to become higher as the grain size of sintered body becomes finer. It is found, however, that as the primary particle diameter of conventional fine α-alumina powder becomes smaller the aggregate force of the powder becomes larger and this enhances tendency to provide the secondary particles which are produced by aggregating the primary particles.
Therefore even when the conventionally known α-alumina powder with small primary diameter is used, there remains voids in the sintered body since the conventional fine powder aggregates to form the secondary particles having larger particle diameter. Consequently it has been difficult to obtain a sintered body with high density. It has been desired to obtain an α-alumina powder which is small in particle size and is not susceptive to aggregation, i.e. weak in aggregate force in order to obtain a sintered body with a high degree of density.
As a method for manufacturing a fine α-alumina powder it is disclosed, for instance, in JP-A-62-128918 a method in which α-alumina particle is added to an alumina precursor such as an alumina-gel as a seed crystal and then the mixture is calcinated at from 900 to 1350° C. followed by milling to afford α-alumina powder with an average primary particle diameter of 70 nm. When a sintered body is provided using thus obtained α-alumina powder, the density of the sinter body is as low as 3.65 g/cm3 (relative density 92%) and it is found that the α-alumina powder according to the above described method can not satisfactorily be used as the material for making sintered body.
Another method of manufacturing a fine alumina particles is disclosed in JP-A-03-080106 in which aluminum trichloride is calcinated at a temperature ranging from 1200 to 1700° C. Although the primary diameters of the resulting fine alumina particles will range from 50 to 200 nm, the content of α-phase alumina in the alumina powder is as low as 70% and can not satisfactorily be used as an abrasive.